This invention relates to organic conductors and semiconductors which fall into the group of polymeric conductors. As is well known, such conducting polymers appear in some respects like common synthetic resinous materials, but unlike such common materials (plastics), conducting polymers defy conventional melt-processing, cannot be compacted, whether molded or extruded, in the usual ways, nor deposited as a continuous film from solution, and are far from stable in air even at ambient temperature conditions. As long as a polymer is formed by electrodeposition on an electrode its conductivity may be said to be fair depending upon the particular application for which the polymer is sought. But a polymer which defies compaction into a shaped article, places severe limitations upon its use. Because a compactable conductor (the term "conductor" as used herein includes semiconductors) is far more versatile in its applications, the problem was to find a compactable polymer.
Tinkering with the structure of conducting polymers to improve their processability, for example by introducing substituents, generally results in degradation of their already low conductivity, consistent with the belief that conductivity is along the polymer chains. Low conductivity in the range from about 10.sup.-5 to about 10.sup.-2 ohm.sup.-1 cm.sup.-1 (reciprocal ohms/cm is hereafter designated "S/cm" for convenience) places a conductor in the category of semiconductors, while conductivity in the range from about 10.sup.-2 to about 10.sup.2 S/cm and above places it in the category of relatively good conductors. Of course, such "low conductivity" is referred to as such only in relation to the high conductivity of metals, but this low conductivity is sufficiently high for a variety of applications, for example, as polymer films on electrodes, as is described in articles titled "Polymer Films on Electrodes. 6. Bioconductive Polymers Produced by Incorporation of Tetrathiafulvalenium in a Polyelectrolyte (Nafion) Matrix" by Henning, T. P., White, H. S., and Bard, A. J., J. Am. Chem. Soc., 103 3937-38, (1981 ); and, "Polymer-Modified Electrodes: A New Class of Electrochemical Materials", by Kaufman, F. B., Schroeder, A. H., Engler, E. M., and Patel, V. V. Appl. Phys. Lett., 36(6), 422-5, (1980).
Poly(2,5-pyrrole), also referred to herein as "PP" for brevity, in which the --NH-- group links sequences of conjugated double bonds, is normally an insulator, that is, has a conductivity less than about 10.sup.-10 ohm.sup.-1 cm.sup.-1 and is totally insoluble in known solvents. It is known however, that electrochemically polymerized PP has good conductivity, but coupled with its melt-processing-resistance and the poor integrity of PP film so formed, the metamorphosis of PP into a practical organic polymer conductor poses a formidable problem. Moreover, it is generally known that providing substituents on the pyrrole monomer does not improve the conductivity of PP. This is not undesirable with respect to tailoring a semiconductor but contraindicates a logical course of action for tailoring a relatively good conductor.
The interest in modification of electrode surfaces by covalently attaching an organic monolayer or by depositing a polymer film spurred the electrochemical polymerization of pyrrole under controlled conditions as reported in "Electrochemical Polymerization of Pyrrole" by Diaz, A. F. et al in J. C. S. Chem. Comm. 1979, 14, 635. The films may be prepared in a variety of aprotic solvents but are totally insoluble in known solvents including acetonitrile (MeCN), methylene chloride and propylene carbonate. Subsequently, PP with p-type conductivity of 100 S/cm was prepared which were stable in air. These films were prepared from MeCN solution using a tetraethylammonium tetrafluoroborate electrolyte, as described in "Organic Metals: Polypyrrole a Stable Synthetic `Metallic` Polymer" by Kanazawa, K. K. et al in J. C. S. Chem. 1979, 15, 854. Because the polymer film remains on the electrode surface as it is generated, the ability of the PP film to conduct is critical for the continuation of the reaction forming the film. Considerations related to the forming of the film, the electroactive behavior of thin films, and other details are discussed in "Electrochemical Preparation and Characterization of Conducting Polymers" in Chemica Scripta., 1981, 17 145-148.
Particularly noteworthy is that PP requires no dopant because it is naturally positively charged indicating it already has an electron removed during polymerization. Even more noteworthy is that PP films which were formed with various substituents on the N-atom were also totally insoluble. The magnitude of this limitation may only be gauged in terms of the limited application of any polymer which requires that it be electrodeposited on an electrode as a film, and which must be used in no other but the film form. At best, film of known PPs is difficulty powdered, and such powders as are formed cannot be pressed into a coherent shaped article even at 100,000 psi.
Despite knowing that substituents on the pyrrole ring would diminish conductivity, and recognizing that the 2- and 5- positions must necessarily remain open if the substituted pyrrole is to be polymerized, I surmised that the possibility of making compactable PP might hinge upon my finding the correct combination of substituents on the pyrrole nucleus. I further hoped that such a 3- and/or 4- substituted pyrrole would lend itself to electrochemical polymerization with an appropriate electrolyte which might favorably affect the solubility of the polymer formed. As will presently be evident, I was successful in the preparation of compactable polypyrroles, but they were essentially insoluble in known solvents.
Of particular interest was that, knowing a substituted pyrrole can be made in which the substituents in the 3- and 4- positions were cyclized ("3,4-cyclized pyrrole"), the fact is that the strain on the molecule is great. Such a strained molecule is unlikely to withstand a reduction reaction without affecting its strained condition. This is substantiated by the fact that such bicyclo fused-ring pyrroles are not known except for those disclosed in articles titled "Synthesis of Isobenzofuran-4,7-quinone and Isoindole-4,7-quinone" by Cragg et al, J. Chem. Soc. Perkin Trans. 1975, (14) 1339-42; and, "Stable Quinoid Derivatives of Isobenzofuran and Isoindole" by Giles et al, J. Chem. Soc. Chem. Commun. 1975, (7) 260.
Making such compounds by the method described in the foregoing references is impractical. Further, it will be noted that in a compound having the foregoing structure, there are oppositely disposed carbonyl groups in the ring which has a single double bond, and even if a method is found to reduce the compound, this double bond would still endure.
Since this product of interest is the PP polymer, it is necessary that the N-adjacent carbon atoms (that is, the C atoms in the 2- and 5- positions) be left open. There was nothing to suggest that if a polymethylene group could bridge (that is, be cyclized with) the 3- and 4- C atoms of the pyrrole ring, the resulting bicyclo compound would, when polymerized, because of the polymethylene bridge, be transformed from incompactable PP to compactable PP, if indeed the substituted PP proved to be electrically conductive. Nor was there anything to suggest that the critical polymethylene bridge might, with the proper choice of additional substituents on the bridge, lend the PP both solubility and enhanced conductivity.
I polymerized several compounds I made for the specific purpose of studying the electronic effects of substituents on conductivity, and it was only by chance that I discovered the criticality of the polymethylene bridge. This led me to seek and find an appropriate method of preparing a cyclic 3,4-polymethylenepyrrole, particularly tri- and tetramethylene pyrroles (hence "T-P" for brevity), some of which were particularly suitable for electropolymerization, and some of which are useful microbicides.
Accordingly, I cast about for a method of forming the polymethylene bridge between the 3- and 4- C atoms of the pyrrole ring and unexpectedly discovered that a cycloalk-2-enone would react with tosylmethylisocyanide so as to form a pyrrole ring resulting in a bicyclo fused ring carbonyl compound. Such a ring closure was known to occur with acyclic esters, ketones and nitriles in the van Leusen procedure (see "A New and Simple Synthesis of the Pyrrole Ring System from Michael Acceptors and Tosylmethylisocyanides" by van Leusen, A. M. et al, Tetrahedron Letters, No. 52, pp. 5337-5340, 1972). That there is a peculiarity about the reaction with a cycloalk-2-enone stems from the discovery that the presence of the alkylene ring does not prevent the cyclization of the vinyleen C atoms to yield a bicyclo fused-ring compound.
Further, though I was aware that a "Michael condensation" between a cycloalk-2-enone and certain compounds was feasible with an appropriate catalyst, there was no indication that the cycloalk-2-enone might behave as it does, because one would expect that the monocyclic Michael condensation product would be unlikely to form a strained bicyclo fused-ring product. The peculiarity of this reaction as it does actually occur, permits the cyclization of the condensation product's vinylene C atoms into a pyrrole ring. See "The Michael Reaction", Organic Reactions Vol. 10, 187 et seq., John Wiley & Sons (1959).
Having succeeded in making the desired 3,4-cyclized pyrrole from a cycloalk-2-enone which was a Michael acceptor, I reduced the carbonyl reaction product and electropolymerized it. Finally, I tested the PPs for electrical properties, and most of all, for compactability, and/or solubility in available solvents.